A Pair in which the value is a container, is mutable. And therefore cannot be a value type:
$ 6 'my $p = a => my $ = 42; say $p.WHICH; $p.value = 666; say $p.WHICH' Pair|140196969054016 Pair|140196969054016 Note if the value is *not* a container: $ 6 'my $p = a => 42; say $p.WHICH; $p.value = 666; say $p.WHICH' Pair|Str|a|Int|42 Cannot modify an immutable Int in block <unit> at -e line 1 I think this is ENOTABUG > On 15 Aug 2016, at 15:56, Zefram (via RT) <perl6-bugs-follo...@perl.org> > wrote: > > # New Ticket Created by Zefram > # Please include the string: [perl #128948] > # in the subject line of all future correspondence about this issue. > # <URL: https://rt.perl.org/Ticket/Display.html?id=128948 > > > > Pair.WHICH has special behaviour if the pair's value is a container: > >> my $a; my $b = :a($a); my $c = :a($a); $a = 3; for $b, $c { say (.WHICH, >> .perl) } > (Pair|47082244733376 :a(3)) > (Pair|47082244733472 :a(3)) > > As you can see, both pairs are behaviourally equivalent, in that they > have the same key and refer to the same value container, thus both seeing > the change in the value contained in it. But they get different .WHICH > values, falsely suggesting that they're distinct. (=== follows .WHICH > in falsely indicating that they're distinct.) > > The container case of .WHICH is using the default .WHICH that reflects the > physical identity of the Pair objects. To get the intended equivalence, > Pair.WHICH needs to reflect the key and the identity of the container held > in the value slot. I think that just removing the branch of Pair.WHICH > for the container case would be correct. > > -zefram
